1. Field of the Invention
The present invention is directed to a method for determining the position of at least one local antenna in an examination space of a magnetic resonance system, of the type wherein magnetic resonance signals emitted by a body arranged in the examination space are received by the local antenna encoded as to location in at least one coordinate direction, and wherein intensity values that are spatially resolved in the coordinate direction are generated from the received magnetic resonance signals, and wherein the position of the local antenna is determined from a curve of the intensity values along the coordinate direction, the intensity values possibly having been subjected to a post-processing.
2. Description of the Prior Art
Magnetic resonance tomography is a known technology for acquiring images of the inside of the body of a living examination subject. For implementing magnetic resonance tomography, a basic field magnet generates a static, relatively homogeneous basic magnetic field. During the registration of magnetic resonance images, rapidly switched gradient fields, which are generated by gradient coils, are superimposed on this basic magnetic field. Radio-frequency transmission antennas are used to emit radio-frequency pulses for triggering magnetic resonance signals into the examination subject arranged in the examination space of the magnetic resonance system. The location-encoded magnetic resonance signals triggered by these radio-frequency pulses are picked up by radio-frequency reception antennas. The magnetic resonance images of the examined subject region of the examination subject are produced on the basis of these magnetic resonance signals received with the reception antennas. Every picture element in the magnetic resonance image thereby corresponds to an intensity value of a received magnetic resonance signal of a small body volume. Whole-body radio-frequency antennas usually are employed as radio-frequency transmission antennas. These whole-body radio-frequency antennas also are suitable as reception antennas for the magnetic resonance signals. They exhibit a very uniform sensitivity profile but leads to a signal-to-noise ratio that is unfavorable for many applications. Local antennas are employed for improving the signal-to-noise ratio. These are antennas that are adapted to the size of the body region to be examined. For example, local antennas are known that are permanently installed in the patient table (bed) or latched to fixed positions at the patient bed for examining the spinal column or for examining the head. In addition to these local antennas that can be positioned in a fixed fashion, freely movable local antennas are also utilized in many applications; the operator of the magnetic resonance system, for example, merely places these onto the body region to be examined.
It is important for a high signal-to-noise ratio in magnetic resonance exposures with such local antennas that the antenna be located as close as possible to the imaging region of the body. When a number of local antennas are available, then only one of these antennas that is located closest to the region to be examined should be activated for the reception during the measurement under certain circumstances. Artifacts such as, for example, folds in the magnetic resonance image are avoided or at least reduced in this way. This, however, requires exact knowledge of the position of the local antennas in the examination space relative to the body region to be examined. However, there is no fixed reference to the magnet system of the magnetic resonance system in employment of local antennas or local coils that can be freely positioned, for example surface coils.
European Application 0 844 488 discloses a method for acquiring the position of a medical instrument equipped with local antennas in an examination space of a magnetic resonance system. To this end, the medical instrument has at least two positioning elements at the local antennas that are provided with magnetic resonance signal sources. The orientation of these positioning elements is acquired with a magnetic resonance pre-measurement and is mixed into the magnetic resonance image of the subsequent imaging magnetic resonance measurement of the body region under examination. The magnetic resonance signal sources thereby enable the determination of the position of the local antennas integrated in the instrument relative to the region under examination. If there is an unfavorable selection of the pick-up region, however, these signal sources can fold into the diagnostic image. A separate transmission and reception branch that is sensitive to a different frequency would have to be available for the pre-measurement information in order to avoid this folding.
U.S. Pat. No. 6,034,529 likewise discloses a method for determining the position of a local antenna in an examination space of a magnetic resonance system that is implemented similar to that of European Application 0 844 488. In this technique, too, the local antenna contains an additional magnetic resonance signal source that can be seen in a magnetic resonance image acquired with a suitable measurement sequence. In this publication, however, the signal source is selected such that its gyromagnetic constant deviates sufficiently from the gyromagnetic constant of protons. This results in the additional magnetic resonance signal source is not visible in the magnetic resonance image of the actual diagnostic measurement. However, this method also requires the use of special local antennas that form the corresponding magnetic resonance signal sources.
German PS 196 53 535 discloses a method for determining the position of at least one local antenna in an examination space of a magnetic resonance system. Magnetic resonance signals emitted by a body arranged in the examination space are received location-encoded in at least one coordinate direction by the local antenna, and intensity values that are spatially resolved in the coordinate direction are generated from the received magnetic resonance signals. Simultaneously, the emitted magnetic resonance signals are received by an antenna that has a uniform sensitivity profile and are converted into corresponding spatially resolved intensity values. By normalization onto the intensity values obtained with the antenna having a uniform intensity distribution, normalized intensity values are formed from the intensity values obtained with the local antenna. A maximum value that indicates the position of the local antenna in a coordinate direction is identified from the curve of these normalized intensity values over the coordinate axis. This method thus does not require the use of additional magnetic resonance signal sources. Due to the noise components in the measured signals, however, the determination of the maximum of this curve is afflicted with error, so that a spatial smoothing or filtering of the intensity values must be implemented for enhancing the precision of the position identification. Such a filtering or smoothing, however, can still lead to imprecisions in the position identification. Moreover, an exact position identification is possible only with this technique when the middle of the local antenna is situated in the exposure volume. When this is not the case, then the position of this antenna is incorrectly identified.
An object of the present invention is to provide a method for determining the position of a local antenna in an examination space of a magnetic resonance system that enables an automatic determination of the position with high precision.
This object is achieved in accordance with the invention in a method wherein magnetic resonance signals emitted by a body arranged in the examination space are received location-encoded in at least one coordinate direction by the local antenna, and intensity values that are spatially resolved in the at least one coordinate direction are generated from the received magnetic resonance signals. The position of the local antenna in the coordinate direction is ultimately determined from the curve of the intensity values (subjected to as needed post-processing) along the coordinate direction. This determination is made by a curve fit, i.e. the adaptation or matching of a function to the curve of the intensity values along the coordinate direction. A function that can at least be approximated or adapted to the shape of a sensitivity profile of the local antenna is utilized for this. After the adaptation of this function to the curve, the position of the maximum of the function on the coordinate axis is utilized as the position of the local antenna.
The position of local antennas can be determined with high precision in at least one freely prescribable coordinate direction with the present method. The method is less susceptible to error, particularly less sensitive to noise influences, compared to the method according to German PS 196 53 535. A smoothing or filtering of the curve of the intensity values for enhancing the precision of the position identification is not required in the inventive method. A very dependable determination of the position of the antenna is achieved with high precision by taking the reception profile of the local antenna into consideration in the curve fit. The position of the antenna can also be correctly identified on the basis of this curve fit when the middle of this antenna is situated outside the exposure volume. Another considerable advantage is that the width of the sensitivity profile of the local antenna in the body under examination can be determined with respect to the respective coordinate direction from the width of the adapted function, for example from the half-width value or the 1/e width, as ensues in a preferred embodiment of the present method. The knowledge of this width of the sensitivity profile, also referred to below as the illumination width or illumination profile, is necessary in order to avoid artifacts which may arise in the magnetic resonance image.
A Gauss function, which is approximated to the sensitivity profile of many antenna or coil types preferably is utilized as adaptation or: matching function. Other functions can be utilized insofar as they can be at least approximately adapted to the sensitivity profile of the antennas.
The present method is preferably implemented in at least two coordinate directions, so that the position of the local antenna can be determined in a plane or in three-dimensional space.
Particular advantages are achieved from the position determination when a number of local antennas are simultaneously utilized in the magnetic resonance system, for example in the form of an antenna array. For example, the positions of the local antennas are employed for selecting one of the local antennas that supplies the best possible signal-to-noise ration, and thus the best resolution for a specific body region. This antenna selection can also ensue automatically. The determination of the position of a number of local antennas preferably ensues simultaneously by the magnetic resonance signals being simultaneously received by these local antennas and evaluated as described above for the determination of the positions of the respective local antennas. This, however, requires that the magnetic resonance system have a number of signal processing channels connected to the individual local antennas.
In another embodiment of the inventive method, the magnetic resonance signals are simultaneously received by an antenna with a uniform sensitivity profile, for example the whole-body antenna used for the emission of radio-frequency signals, and intensity values that are spatially resolved in coordinate direction are generated therefrom. The intensity values of the magnetic resonance signals received with the at least one local antenna are then normalized onto the intensity values of the antenna with the uniform sensitivity profile that are obtained for the same coordinate point. In this way, the influence on the curve of the intensity values over the coordinate axis caused by the body material is eliminated.
The position of the at least one local antenna obtained with the method preferably is mixed into a magnetic resonance overview image of the type usually generated before the implementation of a diagnostic magnetic resonance exposure. The illumination width of the respective local antenna that is determined from the width of the adapted function then can also be displayed in this overview exposure.